The third generation partnership Project, 3GPP, is currently working on standardizing the next generation of mobile communication system denoted Long Term Evolution, LTE, also referred to as Evolved UTRAN or e-UTRAN. LTE is based on a flat architecture compared to 2G and 3G systems such as DSM and WCDMA. Each cell is served by a base station called eNodeB (eNB), and handover between between cells are handled mainly by signaling directly between the eNodeBs and not via any radio network controller. The cell broadcasts an identifying signature, referred to as Physical Cell Identity, PCI, which the user equipments use to identify cells. Since the handover function in LTE is distributed to the eNodeB, each eNodeB keeps a neighbor cell relation list of plausible handover candidates with connectivity information as well as a mapping between the PCI and a unique E-UTRAN cell global identifier, ECGI.
In GSM and in UMTS the neighbor cells, i.e. handover candidates, were configured manually by means of drive tests and performance analysis. This is time consuming and requires a lot of effort.
Thus, manually provisioning and managing neighbor cells in traditional mobile network is a challenging task and it becomes more difficult as new mobile technologies are being rolled out while 2G/3G cells already exist. Therefore, in addition to defining intra LTE neighbor relations for eNodeBs, an operator has to provide neighboring 2G, 3G, CDMA2000 cells as well.
For this reason a new feature called Automated Neighbor Relations, ANR, was introduced in LTE. Thanks to this feature, neighbor cells are added to and removed from the neighbor cell relation list automatically without human intervention. The purpose of the Automatic Neighbour Relation (ANR) functionality is thus to relieve the operator from the burden of manually managing Neighbor Relations (NRs).
In LTE, the ANR function thus resides in the eNodeB and keeps the conceptual Neighbor Relation Table (NRT), also referred to as Neighbor Cell Relation list. Located within ANR, a Neighbor Detection Function finds new neighbors and adds them to the NRT. ANR also contains a Neighbor Removal Function which removes outdated neighbor cells relations. The Neighbor Detection Function and the Neighbor Removal Function are implementation specific.
An existing Neighbor cell Relation (NR) from a source cell to a target cell means that eNodeB controlling the source cell knows the E-UTRAN Cell Global Identifier, ECGI/CGI, and Physical Cell Identifier (PCI) of the target cell and has an entry in the NRT for the source cell identifying the target cell.
For each cell, the eNB keeps an NRT. For each NR, the NRT contains the Target Cell Identifier (TCI), which identifies the target cell. For E-UTRAN, the TCI corresponds to the E-UTAN Cell Global Identifier (ECGI) and Physical Cell Identifier (PCI) of the target cell.
The ANR function relies on cells broadcasting their identity on global level, E-UTRAN Cell Global Identifier (ECGI) and allows the O&M system to manage the NRT. The O&M system can add and delete neighbor relations, NR. It can also change the attributes of the NRT. The O&M system is informed about changes in the NRT.
The basic functionality of ANR is illustrated in the cellular radio network 100′ shown in FIG. 1. An eNodeB serving a cell with ANR function instructs user equipments to perform measurements on neighbor cells as a part of the normal call procedure. The eNodeB may use different policies for instructing the UE to do measurements, and when to report them to the eNodeB. If a user equipment 120′ in Cell A, by measurements, detects that the signal received from a neighbor cell B is strong, it reports the measurement result to the eNodeB 110′ of Cell A in step 1. However at this stage neither the user equipment 120′ nor the eNodeB 110′ knows the unique Global Cell Identity (Global-CID), which is an element of ECGI, of Cell B. For this reason the eNodeB 110′ orders the mobile in step 2 to read the broadcast channel BCCH of cell B and obtain the Global-CID. The user equipment 120′ gets that information in step 2b) by reading the BCCH and sends the information to the eNodeB 110′ of cell A in another measurement report in step 3. In addition, the user equipment reports the tracking area code and all Public Land Mobile Network, PLMN, IDs that have been detected. The eNodeB 110′ of cell A stores all this information in its ANR database and configures cell B as a neighbor to cell A, i.e., cell B is listed as a handover candidate in the NRT.
However, it may happen that in the network, for different reasons, a base station, e.g. an eNodeB, of a cell broadcasts its coverage far beyond its planned area. This could cause interference to other user equipments communicating outside the planned coverage area of said base station. Operators may detect such behaviors for example by monitoring drop calls statistics, or by performing some drive test, or by analyzing call traces triggered remotely from Operation and Maintenance Center, OMC.
Moreover, if such distant cell is added to the neighbor relation table in an eNodeB as a candidate target cell, problems would arise if a user equipment was handed over from said eNodeB to the distant cell.